Abstract:

Data from twenty Brazos River stratigraphic sections of the K–Pg event deposit are compiled to provide a record of changing depositional processes on the Texas continental shelf that left a record of disturbance triggered by the Chicxulub impact. Foraminiferal paleobathymetric depth control of enclosing sediments indicates paleo–water depths of 75–125 m with gradual deepening above the K–Pg boundary. Outcrop-scale observations indicate that the most complete deposits have a basal unit of mudstone clast-bearing mass flows indicative of movement by noncohesive laminar flow. These mudflows were mobilized by impact-generated seismic shaking of seafloor sediment. The material of the mass flows consists of reworked shell-rich mud matrix containing outsize clasts of consolidated mudstone and cemented concretions. The upper parts of the K–Pg event deposit consist of units of high-energy upward-fining sandstones, each with a basal lag layer of spherule-rich coarse sandstone lag overlain by poorly or moderately sorted hummocky cross-bedded sandstone. The last deposited sandstone is capped with a silt-mud suspension settling layer that as much as 10 cm thick where cemented with calcite, but is much thinner elsewhere. The provenance of these K–Pg Brazos River event deposit sediments is offshore marine muds (lower units) and shallow-water sands (higher units) with admixed impact spherules and no identifiable land-derived component. Deposition was the result of secondary processes activated by energy from seismic disturbance or by atmospheric disturbance, with no deposits directly attributable to tsunami wave control.